1. Field of the Invention
The present invention relates to a regional positioning method and apparatus in a wireless sensor network (WSN).
This work was supported by the IT R&D of MIC/IITA. [2006-S-601-02, development of distributed sensor network system]
2. Description of Related Art
A ubiquitous sensor network (USN) is referred to as a wireless sensor network (WSN) including a sensor node equipped with a sensor capable of catching information from an object or environment where the node is installed. In a USN, information received from a variety of sensors is processed and managed in real time by connecting to an external network through a network. A USN embodies an environment where communications are possible among objects anywhere and anytime regardless of a network, device, and service.
FIG. 1 is a diagram illustrating a general configuration of a USN and WSN.
As illustrated in FIG. 1, the sensor network may include a sensor node 110, sensor field 120, sink 130, and gateway 140. The sensor node 110 includes sensors and a communication module. The sensor recognizes information about an object and environment. The sensor field 120 includes a set of sensor nodes 110. The sink 130 receives information collected in the sensor field 120. The gateway 140 performs routing with respect to information transmitted from the sink 130 and transmits the information to a management server 160 through a broadband communication network 150. The sensor network may be connected to an existing infrastructure such as those used for satellite communications, wireless local area network (LAN), Bluetooth, and wired Internet, through the gateway 140.
FIG. 2 is a diagram illustrating a general configuration of a sensor node in a WSN.
As illustrated in FIG. 2, the sensor node includes a central processing unit (CPU) and memory 210, communication module 220, sensor module 230, signal processing module 240, and software 250, and a battery/energy source 260 to provide power to the sensor node. Due to a sensor node's characteristic that the sensor node is operated with limited battery power, low power consumption is critical when designing sensor nodes. Accordingly, a hardware circuit is usually configured using low power elements, for example, a low capable CPU, and a low power communication element and peripheral circuit.
FIG. 3 is a diagram illustrating a general positioning method in a sensor network through sensing a targeted object.
As illustrated in FIG. 3, in the positioning method, a sensor network field includes a sensor node 310 where a sensor 330 and analog to digital (A/D) or digital to analog (D/A) converter 320 are installed. The sensor 330 senses characteristics of a target 340, for example, movement, material, weight, and the like. When it is sensed that the target 340 enters a sensing area 380 as illustrated in a location 350, location information of the sensed target 340 is transmitted to a sink or gateway 370 through a multi-hop communication, and thus a location of the target 340 may be recognized.
In a traditional positioning method, a location of a target may be detected when the target enters a sensing area, due to a characteristic of a sensor network. However, when the target does not clearly enter the sensing area or when a valid sensing area is not appropriately distributed, the location of the target may not be detected.
Also, in a traditional positioning method, a location of a target may be detected using a calculation method based on a radio signal strength as opposed to the above-described method using the entry to the sensing area. The method is divided into a scheme utilizing measured distance information and a scheme without utilizing distance information. In the scheme utilizing the distance information, a distance is measured between a targeted node with respect to its location and each of at least three anchor nodes whose locations are already known. After measuring the three distance values, a triangulation is performed to find out the location. In this instance, the distances may be measured by a Time of Arrival (ToA), Time Difference of Arrival (TDoA), Received Signal Strength (RSS), and the like. The scheme without utilizing distance information includes a centroid method, Approximate Point In Triangulation (APIT), and the like. A distance-based method in a multi-hop sensor network may overcome a disadvantage where an error may be spread to the sensor network. In the centroid method, location information of regularly-arranged anchor nodes is transmitted to neighboring nodes, the neighboring nodes receiving the location information compare strengths of signals transmitted from the anchor nodes, and thus a location of each of the neighboring nodes may be predicted.
However, in the centroid method, traffic load in the sensor network may vary depending on a position of a locating engine performing the calculation. For example, when the locating engine is placed in the sink of the sensor network, multiple signal strength information collected based on a targeted node is transmitted via multi-hop. Accordingly, the additional traffic is loaded in the sensor network. When the locating engine is placed in a target, power is consumed to receive radio signal strength information from neighboring nodes and calculate the radio signal strength information, which affects a network lifetime. Accordingly, hardware solution is preferred to software when the locating engine is located in the target.